There is return on investment even if it is not monetarily. Users or reCAPTCHA or Duolingo also do work for for-profit companies as well.

From the article:

> “Some people ask ‘Why do you work for free? Why do you spend so many hours a week doing this mapping?’ ” Hidalgo said. “You feel good about knowing that the people who drive around everyday save time because you are investing your time in fixing the roads and doing this mapping.”

I wouldn't do it either, but it sounds like they are solving a problem for the people around them.

(A simple way to look at it is that if Google/Waze manage to extract some amount $X in profit because of their effort, it is very likely that the people using the product derived a benefit greater than $X from that same effort. It's also possible that some benefit is delivered to the users from things that Google/Waze doesn't manage to profit off of.)

.. and before CDDB, it was Viewpoint Data Labs .. its a part of the heritage of the Internet at this point.